6o THE SUN. 



power of gravitation, that its intensity the energy of its 

 pull is less and less as the distance of the thing pulled 

 is greater : and that in a higher proportion. At double 

 the distance, the force of the pull is not halved, but 

 quartered : at triple, it is not a third part, but a ninth. 

 There are mountains in the world five miles high ; that 

 is to say, whose summits are five miles farther from the 

 centre of the earth than the sea-level. If a ton of lead 

 were carried up to the top of such a mountain, though 

 it would still balance another ton, or 2240 weights of a 

 pound each on the scales, then and there ; yet it would 

 not require so great an effort, such an exertion of mus- 

 cular force, to raise and sustain it by five pounds and a 

 half Now, fancy it removed to a height of 94,900,000 

 miles from the earth's surface, and estimating by the 

 same rule its apparent weight, you will find, if you make 

 the calculation, that it would not require more effort to sus- 

 tain it from falling, than Avould suffice to lift one thirty- 

 seventh part of a grain from the surface of the earth. 



(17.) This, then, one thirty-seventh part of a grain, is 

 the force which the earth, placed where the sun is, would 

 exert on our lump of lead. But we have seen that to 

 retain such a lump in such, an orbit requires a pull of i 

 lb. 6 oz. 51 grs. Of course, then, the earth, so placed, 

 would be quite inadequate to retain it from flying off. 

 To do this would require as many earths to pull it as 

 there are thirty-seventh parts of a grain in i lb. 6 oz. 

 51 grs. : that is to say, by an easy sum in arithmetic, 

 356,929; or in round numbers, 360,000. Now, this is 

 equivalent to saying, tliat to do the work which the sun 



